The present invention relates to a semiconductor device, and more particularly to a bonding structure between a semiconductor substrate and a supporting electrode made of molybdenum, tungsten, or the like.
Aluminum solder (containing only aluminum or aluminum as its main component) is frequently employed to bond a semiconductor substrate to an electrode for supporting the semiconductor substrate and for supplying an electric current to the substrate. Further, in some cases, silicon substrates are bonded to each other with the aluminum solder. The reason why aluminum is employed as the brazing solder in the above-mentioned cases is that aluminum is superior to any other brazing solder in electrical conduction and adhesive properties. Also aluminum solder is a hard solder and is inexpensive.
However, when the surface of the silicon substrate which is to be soldered has an n-type conductivity portion, difficulties arise in that aluminum is alloyed with silicon by the heat of the bonding operation. Therefore, a layer having a p-type conductivity is formed in the n-type portion of the surface, and the forward voltage drop (hereinafter referred to as an FVD) of the silicon substrate is increased.
In order to suppress the formation of such a p-type layer in the n-type layer, methods have been employed such as making the aluminum solder thin, interposing between the silicon substrate and the aluminum solder a foil made of antimony which is a pentavalent element, or diffusing a large amount of phosphorus into the n-type portion of the surface. However, such methods have been carried out without any marked effect.
Further, the silicon substrate and the electrode have to be heated to a high temperature during soldering after impurities for determining the conductivity type have already been diffused into the silicon substrate. Accordingly, the distribution of the impurities in the silicon substrate is changed, and a heavy metal which acts as a lifetime killer in the silicon substrate is diffused therein. Thus the silicon substrate is adversely affected by the bonding operation.